1. Field of the Invention
The present invention relates to an apparatus and method of molding a heat-resistant container particularly from a synthetic resin such as polyethylene terephthalate (which will be called "PET").
2. Description of the Prior Art
In general, a synthetic resin thin-walled packaging container known as biaxial stretching blow molded container is formed by positioning an injection-molded or extruded preform having an appropriate temperature for stretching within a mold and stretching the preform in its longitudinal direction corresponding to the longitudinal axis of the container while at the same time expanding the same preform in its lateral direction under the action of a pressurized gas blown into the mold.
Depending on selection of a material used to form the container, however, a problem was raised in that the container deformed when it was filled with a hot content such as a thermally sterilized fruit juice beverage.
To overcome such a problem, a proposal such as the applicant's Japanese Patent Application Laid-Open No. 3-205124 has been made in which the blow molding step to be executed after the temperature of the preform has been regulated is divided into primary and secondary sub-steps. In the primary blow molding sub-step, a primary molding is formed in the desired form of a container. The primary molding is thermally processed to shrink and then subjected to the secondary blow molding sub-step to form the final container.
Such a proposed molding process can provide a heat-resistant container which is improved in mechanical strength through the thermal treatment before the secondary blow molding sub-step.
More particularly, the thermal treatment before the secondary sub-step removes a strain produced at the primary blow molding sub-step or a residual stress due to stretching, and crystallizes the oriented walls to a higher level. This improves the heat resistance of the final product which may be placed under a severe temperature condition in markets.
To attain such a heat-resistant container, it is required that the temperature of the primary molding has been increased sufficiently to improve the crystallinity in the primary molding at its oriented walls.
However, the prior art could not smoothly increase the temperature of the molding since the necessary heat was only transmitted to the molding through radiation within an atmosphere.
Therefore, a long time is required until the temperature of the molding reaches a level that can provide the necessary crystallinity for the molding to have its sufficient heat resisting property. Thus, time for heating or conveying the molding must be prolonged. This may extend the molding cycle or increase the dimensions of the container molding apparatus including the heating conveyor path.